An internal combustion engine, particularly a highly efficient diesel engine is normally provided with an exhaust gas after-treatment system, for degrading and/or removing the pollutants from the exhaust gas emitted by the diesel engine, before discharging it in the environment.
The after-treatment system generally includes an exhaust line for leading the exhaust gas from the diesel engine to the environment, a Diesel Oxidation Catalyst (DOC) located in the exhaust line, for oxidizing hydrocarbon (HC) and carbon monoxides (CO) into carbon dioxide (CO2) and water (H2O), a Lean NOx Trap (LNT), for trapping nitrogen oxides NOx contained in the exhaust gas and is located in the exhaust line and a diesel Particulate Filter (DPF) located in the exhaust line downstream the DOC, for removing diesel particulate matter or soot from the exhaust gas.
To further reduce the emissions content, in particular NOx emissions, normally diesel engines include an exhaust gas recirculation (EGR) system coupled between the exhaust manifold and the intake manifold. This embodiment is also called as high pressure exhaust gas recirculation (HP-EGR). As known, the EGR works by recirculating a portion of an exhaust gas from the engine back to the engine cylinders. In a diesel engine, the exhaust gas replaces some of the excess oxygen in the pre-combustion mixture. Because NOx forms primarily when a mixture of nitrogen and oxygen is subjected to high temperature, the lower combustion chamber temperatures caused by EGR reduces the amount of NOx the combustion generates. More recent embodiments also include a low pressure EGR system (LP-EGR) characterized by a “long route” of the exhaust gases. In this case the additional EGR valve will recirculate the exhaust gases downstream of the after-treatment devices towards the compressor inlet. The operating principle of the LP-EGR is the same of the HP-EGR, with the further advantage that the LP-EGR recirculates exhaust gases at still lower temperature. Of course, an important control parameter of such system is the split of the recirculated exhaust gas mass (the EGR rate) between the HP-EGR route and the LP-EGR route.
Although the combination of HP-EGR and LP-EGR, together with the mentioned after-treatment systems, seems very promising for controlling exhaust emissions, it is necessary to increase the functionalities of the low-pressure EGR systems in view of the application of new homologation cycles. Such cycles, as the new Worldwide Light duty Test Procedure (WLTP) and the Real-life Drive Emissions (RDE), are highly transient cycles. In this way, systems that have been designed to operate under the current New European Driving Cycle (NEDC) certification should increase their operating range without hardware upgrade, with a favorable cost benefit. In fact, the homologation cycles WLTP and RDE will increase quite extensively the operating range under which the emission control must be accomplished. For both HP-EGR and LP-EGR systems this may pose a significant design challenge, as operating limits may be exceeded under certain operating conditions, for different reasons. For example, in some engine operating points, the allowable outlet compressor temperature value may be exceeded, while in some other engine operating points the allowable outlet temperature value of the HP-EGR cooler may be overcome as well. In addition, optimal after-treatment operating parameters may be exceeded, due to an excessive space velocity or a high exhaust gas temperature. In particular, exhaust gas temperature and space velocity at the LNT inlet may exceed their optimal values. Finally, the compressor pressure ratio may overcome surge limitations, under fast acceleration transients.
Therefore a need exists for an improved method of controlling an exhaust recirculation gas system of an internal combustion engine, provided with both a low pressure and a high pressure exhaust gas recirculation system.